Juxtaposition of the strong back-bonding carbonyl ligand and weak back-bonding acetonitrile ligand in binuclear iron complexes

Abstract

The binuclear iron complexes (CH3CN)2Fe2(CO) n (n = 7, 6), containing both the strongly back-bonding CO ligand and the weakly back-bonding acetonitrile ligand, have been investigated by density functional theory. The acetonitrile ligands are always found to be terminal in such structures. Heptacarbonyl structures are found at similar energies, with both acetonitrile ligands bonded to the same iron atom or with each acetonitrile ligand bonded to a different iron atom. Various combinations of bridging and semibridging CO groups are found in these (CH3CN)2Fe2(CO)7 structures approaching triply (bridged + semibridged) structures for the structures with symmetrically distributed acetonitrile ligands. The latter structures thus resemble the well-established triply bridged structure for the related binary iron carbonyl Fe2(CO)9. For the hexacarbonyl (CH3CN)2Fe2(CO)6, both triplet unbridged structures and singlet doubly bridged structures are found. The triplet (CH3CN)2Fe2(CO)6 structures have slightly lower energies relative to the singlet structures. The acetonitrile ligands in (CH3CN)2Fe2(CO) n (n = 7, 6) are always found to be terminal ligands. Various combinations of bridging and semibridging CO groups are found in these (CH3CN)2Fe2(CO)7 structures approaching triply (bridged + semibridged) structures for the structures with symmetrically distributed acetonitrile ligands. For the hexacarbonyl (CH3CN)2Fe2(CO)6, both triplet unbridged structures and singlet doubly bridged structures are found.